1. Field of the Invention
The present invention relates to polyimide materials having improved optical transparency and low absolute thermo-optic coefficient (|dn/dT|) that are useful in optically transparent fiber composites (OTFCs), optically transparent ribbon composites (OTRCs), and optical communication devices.
2. Description of Related Art
The thermo-optic coefficient (thermally induced change in refractive index, dn/dT) of polymeric materials has attracted much attention due to their suitability for use in transparent composites (e.g., OTFCs or OTRCs) and optical communications. One type of transparent composite is made up of glass fibers or ribbons bound together with a polymeric material acting as a binder. Transparency in these composites is achieved by matching the refractive index (n) of the glass fibers or ribbons to that of the polymeric material. One problem encountered in such composites is that changes in temperature result in changes of the refractive index for each material, causing them to become mismatched and thereby reducing optical clarity of the composite. For example, the dn/dT of glass is between about 1 to 10×10−6/° C. while the dn/dT for most polymers is between about (−100) to (−300)×10−6/° C. Past efforts at creating OTRCs have used epoxy binders with dn/dT values of about −200×10−6/° C. However, a significant improvement in transparency across a range of working temperatures (e.g., −40° to 125° F.) could be achieved if a polymer matrix material with an absolute thermo-optic coefficient (|dn/dT|) of less than about 100×10−6/° C. and more preferably, less than about 50×10−6/° C., could be developed.
The thermo-optic coefficient is also important in optical communications, especially in optical devices. Polymer optical devices are easier to process and more cost-effective than those made of inorganic materials, such as glass, which are fragile, expensive, and time-consuming to fabricate. In addition, the larger |dn/dT| of polymers over glass favors less power consumption and higher speed response for the optical devices. Therefore, intense efforts have been put forth to investigate the thermo-optic coefficient of a number of polymers, including PMMA, epoxy, polystyrene, and polyimides.
The refractive index of solid polymers is determined to a large extent by two factors: the inherent molecular composition of the polymer and the density of the molecules. When the ambient temperature changes, the resulting change in refractive index is attributable to a change in material density since the composition remains the same. Unlike glasses, polymeric materials tend to expand with temperature, which leads to a decrease in refractive index. Thus, polymers with a rigid structure are advantageous in reducing the thermo-optic coefficient. However, rigid polymers usually have poor solvent solubility, leading to poor processability. Thus, there remains a need for polymers having sufficient rigidity without sacrificing solubility and processability. There is also a need for polymers having improved optical transparency throughout the optical wavelength range (400-700 nm).